Self-capacitive touch sensing circuit and noise suppressing method applied to self-capacitive touch panel

ABSTRACT

A self-capacitive touch sensing circuit including an operational amplifier, an internal capacitor, a first switch and a second switch is disclosed. A first input terminal and a second input terminal of operational amplifier are coupled to a capacitor and ground respectively and an output terminal of operational amplifier outputs an output voltage. The internal capacitor is coupled between the output terminal and first input terminal of operational amplifier. One terminal of first switch is coupled to a first external charging voltage and another terminal of first switch is coupled between the capacitor and the first input terminal. The first external charging voltage is higher than the second external charging voltage. The first switch and second switch are switched according to a specific order, so that the first external charging voltage or second external charging voltage will charge the capacitor.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a touch panel, especially to a self-capacitivetouch sensing circuit and a noise suppressing method applied to aself-capacitive touch panel.

2. Description of the Prior Art

In general, the self-capacitive touch panel can use a design of in-cellstructure or on-cell structure to realize a thin self-capacitive touchpanel.

As to the self-capacitive touch sensing circuit used in prior art, forexample, a shown in FIG. 1, the conventional self-capacitive touchsensing circuit charges the capacitor Cext to a first external chargingvoltage V1 under a first phase, so that the charge amount stored in thecapacitor Cext is a product of the capacitor Cex and the first externalcharging voltage V1.

However, in the in-cell or on-cell self-capacitive touch panel, theself-capacitive touch sensing circuit will be closer to the panel andeasier to be interfered by the noise generated by reversing liquidcrystals. As shown in FIG. 2, the upper curve is a frequency responsecurve obtained under a condition of having an external noise of 10 KHZfrequency, and the lower curve is a frequency response curve obtainedunder a condition of no external noise. It can be found from the lowercurve that the signal part is located at DC level; it can be found fromthe upper curve that the upper curve includes not only the signal partlocated at DC level but also the noise part having a frequency of 10KHZ. Since the frequency of the noise part and the frequency of thesignal part of the upper curve are too close, it is hard for theself-capacitive touch sensing circuit to use a simple low-pass filter tofilter the noise part out.

In addition, because the self-capacitive touch sensing circuit will befarther from the finger signal source, the signal-to-noise ratio of thetouch sensing signal and the touch sensing performance of theself-capacitive touch panel will become poor. It will be necessary touse additional hardware to increase the signal-to-noise ratio of thetouch sensing signal; therefore, the manufacturing cost of theself-capacitive touch panel fails to be reduced.

SUMMARY OF THE INVENTION

Therefore, the invention provides a self-capacitive touch sensingcircuit and a noise suppressing method applied to a self-capacitivetouch panel to solve the above-mentioned problems.

An embodiment of the invention is a self-capacitive touch sensingcircuit applied to a self-capacitive touch panel. In this embodiment,the self-capacitive touch sensing circuit is used for sensing acapacitance variation of a capacitor when the self-capacitive touchpanel is touched and suppressing an external environmental noise. Theself-capacitive touch sensing circuit includes an operational amplifier,an internal capacitor, a first switch and a second switch. Theoperational amplifier has a first input terminal, a second inputterminal and an output terminal, wherein the first input terminal iscoupled to the capacitor and the second input terminal is coupled toground, and the output terminal outputs an output voltage. The internalcapacitor is coupled between the output terminal and the first inputterminal of the operational amplifier. The first switch has a terminalcoupled to a first external charging voltage and another terminalcoupled between the capacitor and the first input terminal. The secondswitch has a terminal coupled to a second external charging voltage andanother terminal coupled between the capacitor and the first inputterminal. The first external charging voltage is higher than the secondexternal charging voltage; the first switch and the second switch areswitched according to a specific order, so that the first externalcharging voltage or the second external charging voltage charges thecapacitor.

In an embodiment, the another terminal of the first switch and theanother terminal of the second switch are both coupled to a node betweenthe capacitor and the first input terminal.

In an embodiment, the self-capacitive touch sensing circuit furtherincludes a third switch coupled between the node and the first inputterminal.

In an embodiment, the self-capacitive touch sensing circuit furtherincludes an analog-to-digital converter coupled to the output terminalof the operational amplifier and a digital signal processor coupled tothe analog-to-digital converter.

In an embodiment, the self-capacitive touch sensing circuit furtherincludes a first polarity unit and a second polarity unit. The firstpolarity unit is coupled between the output terminal of the operationalamplifier and the analog-to-digital converter and used for receiving theoutput voltage having analog form from the output terminal of theoperational amplifier and then outputting the output voltage to theanalog-to-digital converter by maintaining a polarity of the outputvoltage. The second polarity unit is coupled between the output terminalof the operational amplifier and the analog-to-digital converter andused for receiving the output voltage having analog form from the outputterminal of the operational amplifier and then outputting the outputvoltage to the analog-to-digital converter by reversing the polarity ofthe output voltage.

In an embodiment, the self-capacitive touch sensing circuit furtherincludes a first polarity unit and a second polarity unit. The firstpolarity unit is coupled between the analog-to-digital converter and thedigital signal processor and used for receiving the output voltagehaving digital form converted from analog form by the analog-to-digitalconverter and then outputting the output voltage to the digital signalprocessor by maintaining a polarity of the output voltage. The secondpolarity unit is coupled between the analog-to-digital converter and thedigital signal processor and used for receiving the output voltagehaving digital form converted from analog form by the analog-to-digitalconverter and then outputting the output voltage to the digital signalprocessor by reversing the polarity of the output voltage.

In an embodiment, the self-capacitive touch sensing circuit furtherincludes another digital signal processor. The another digital signalprocessor is coupled to the first polarity unit and the second polarityunit respectively and used for receiving the output voltage maintainingpolarity from the first polarity unit and receiving the output voltagereversing polarity from the second polarity unit respectively.

In an embodiment, under an odd-numbered phase, the first switch isconducted but the second switch and the third switch are not conducted,then the first external charging voltage charges the capacitor, and afirst charge amount stored in the capacitor is a product of acapacitance of the capacitor and the first external charging voltage.

In an embodiment, under an even-numbered phase, the third switch isconducted but the first switch and the second switch are not conducted,then the output voltage is a quotient of the first charge amount dividedby the internal capacitor.

In an embodiment, when the capacitor is touched, a product of a firstoutput voltage variation of the output voltage and the internalcapacitor is equal to a product of the first external charging voltageand the capacitance variation when the capacitor is touched.

In an embodiment, under an odd-numbered phase, the second switch isconducted but the first switch and the third switch are not conducted,then the second external charging voltage charges the capacitor, and asecond charge amount stored in the capacitor is a product of acapacitance of the capacitor and the second external charging voltage.

In an embodiment, under an even-numbered phase, the third switch isconducted but the first switch and the second switch are not conducted,then the output voltage is a quotient of the second charge amountdivided by the internal capacitor.

In an embodiment, when the capacitor is touched, a product of a secondoutput voltage variation of the output voltage and the internalcapacitor is equal to a product of the second external charging voltageand the capacitance variation when the capacitor is touched.

Another embodiment of the invention is a noise suppressing method. Inthis embodiment, the noise suppressing method is applied to aself-capacitive touch panel to suppress an external environmental noisewhen the self-capacitive touch panel senses a capacitance changingsignal generated by touch. The noise suppressing method includes stepsof: (a) instantly sensing a raw band of the external environmentalnoise, wherein the raw band is near a direct current (DC) band of thecapacitance changing signal; (b) selecting a corresponding capacitancedriving modulation coefficient according to a sensing result of step (a)to move the external environmental noise from the raw band to ahigh-frequency band, wherein a frequency of the high-frequency band ishigher than that of the raw band and the DC band; (c) pulling thecapacitance changing signal back to the DC band through a demodulationmechanism to make the capacitance changing signal in the DC bandseparated from the external environmental noise in the high-frequencyband; and (d) using a low-pass filter to filter out the externalenvironmental noise in the high-frequency band to keep the capacitancechanging signal in the DC band.

Compared to the prior art, the invention provides a self-capacitivetouch sensing circuit and a noise suppressing method applied to aself-capacitive touch panel to move a self-capacitive sensing signal toa band having less environmental noise through a driving way ofinstantly adjusting modulation coefficient and then modulate theself-capacitive sensing signal to the DC band and use a simple one-stagelow-frequency filter to filter the self-capacitive sensing signal, sothat the signal-to-noise ratio of the self-capacitive sensing signal canbe effectively increased. The self-capacitive touch sensing circuit andthe noise suppressing method of the invention have the followingadvantages:

(1) Suitable for high noise capacitance sensing environment;

(2) Effectively reducing capacitance driving time;

(3) Effectively reducing entire power consumption for capacitancedriving;

(4) Achieving better touch sensing effect.

The advantage and spirit of the invention may be understood by thefollowing detailed descriptions together with the appended drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 illustrates a schematic diagram of the self-capacitive touchsensing circuit in prior art.

FIG. 2 illustrates frequency response diagrams of the self-capacitivetouch sensing signal obtained by the self-capacitive touch sensingcircuit under the condition with external noise and the conditionwithout external noise respectively in prior art.

FIG. 3 illustrates a schematic diagram of the self-capacitive touchsensing circuit in a preferred embodiment of the invention.

FIG. 4A-FIG. 4C illustrate different embodiments of the self-capacitivetouch sensing circuit respectively.

FIG. 5 illustrates a frequency response diagram of the self-capacitivetouch sensing signal obtained by the self-capacitive touch sensingcircuit and its low-frequency part extending to the high-frequencyregion under the condition with external noise in the invention.

FIG. 6 illustrates schematic diagrams of comparing the self-capacitivetouch sensing signals filtered by the simple one-stage low-pass filterin the invention and the prior art respectively.

FIG. 7 illustrates schematic diagrams of comparing the performancedifference between the self-capacitive touch sensing signals with timein the invention and the prior art respectively.

FIG. 8 illustrates a flowchart of the noise suppressing method inanother preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention is a self-capacitive touch sensingcircuit applied to a self-capacitive touch panel. In this embodiment,the self-capacitive touch sensing circuit is used for sensing acapacitance variation of a capacitor when the self-capacitive touchpanel is touched and suppressing an external environmental noise.

Please refer to FIG. 3. FIG. 3 illustrates a schematic diagram of theself-capacitive touch sensing circuit in this embodiment. As shown inFIG. 3, the self-capacitive touch sensing circuit includes anoperational amplifier OP, a capacitor Cext, an internal capacitor Cf, afirst switch SW1, a second switch SW2 and a third switch SW3.

The operational amplifier OP has a first input terminal, a second inputterminal and an output terminal, wherein the first input terminal iscoupled to the capacitor Cext and the second input terminal is coupledto ground, and the output terminal outputs an output voltage Vo. Thefirst switch SW1 has a terminal coupled to a first external chargingvoltage V1 and another terminal coupled between the capacitor Cext andthe first input terminal of the operational amplifier OP. The secondswitch SW2 has a terminal coupled to a second external charging voltage−V1 and another terminal coupled between the capacitor Cext and thefirst input terminal of the operational amplifier OP. The anotherterminal of the first switch SW1 and the another terminal of the secondswitch SW2 are both coupled to a node. The third switch SW3 is coupledbetween the node and the first input terminal of the operationalamplifier OP. One terminal of the internal capacitor Cf is coupled tothe output terminal of the operational amplifier OP and another terminalof the internal capacitor Cf is coupled between the third switch SW3 andthe first input terminal of the operational amplifier OP.

It should be noticed that the first external charging voltage V1 ishigher than the second external charging voltage −V1. The first switchSW1 and the second switch SW2 are switched according to a specificorder, so that the first external charging voltage V1 or the secondexternal charging voltage −V1 charges the capacitor Cext, but notlimited to this.

Next, different driving methods and capacitance to charge detectionmechanisms used in the invention will be introduced in detail.

(1) The First Driving Method:

Under the odd-numbered phase (e.g., the first phase, the third phase, .. . ), the first switch SW1 is conducted but the second switch SW2 andthe third switch SW3 are not conducted, then the first external chargingvoltage V1 charges the capacitor Cext, and a first charge amount storedin the capacitor Cext will be a product of a capacitance of thecapacitor Cext and the first external charging voltage V1, namely thefirst charge amount stored in the capacitor Cext will be equal to[(Cext)×(V1)].

Under the even-numbered phase (e.g., the second phase, the fourth phase,. . . ), the third switch SW3 is conducted but the first switch SW1 andthe second switch SW2 are not conducted, then the output voltage Vo willbe a quotient of the first charge amount [(Cext)×(V1)] divided by acapacitance of the internal capacitor Cf, namely the output voltage Vowill be equal to [(Cext)×(V1)]/(Cf).

When the capacitor Cext is touched, a product of a first output voltagevariation Δ Vout1 of the output voltage Vo and the internal capacitor Cfwill be equal to a product of the first external charging voltage V1 andthe capacitance variation ΔCfinger when the capacitor Cext is touched,namely the first output voltage variation ΔVout1 of the output voltageVo will be equal to [(ΔCfinger)×(V1)]/(Cf).

If the self-capacitive touch sensing signal is interfered by externalnoises, a product of a third output voltage variation ΔVout3 of theoutput voltage Vo and the internal capacitor Cf will be equal to aproduct of the capacitor Cext and a voltage variation Δ Vnoise caused bynoise interference, namely the third output voltage variation ΔVout3 ofthe output voltage Vo will be equal to [(ΔVnoise)×(Cext)]/(Cf).

(2) The Second Driving Method:

Under the odd-numbered phase (e.g., the first phase, the third phase, .. . ), the second switch SW2 is conducted but the first switch SW1 andthe third switch SW3 are not conducted, then the second externalcharging voltage −V1 charges the capacitor Cext, and a second chargeamount stored in the capacitor Cext will be a product of a capacitanceof the capacitor Cext and the second external charging voltage −V1,namely the second charge amount stored in the capacitor Cext will beequal to [(Cext)×(−V1)].

Under the even-numbered phase (e.g., the second phase, the fourth phase,. . . ), the third switch SW3 is conducted but the first switch SW1 andthe second switch SW2 are not conducted, then the output voltage Vo is aquotient of the second charge amount divided by the internal capacitorCf, namely the output voltage Vo will be equal to [(Cext)×(−V1)]/(Cf).

When the capacitor Cext is touched, a product of a second output voltagevariation ΔVout2 of the output voltage Vo and the internal capacitor Cfwill be equal to a product of the second external charging voltage −V1and the capacitance variation Δ Cfinger when the capacitor Cext istouched, namely the second output voltage variation ΔVout2 of the outputvoltage Vo will be equal to [(ΔCfinger)×(−V1)]/(Cf).

If the self-capacitive touch sensing signal is interfered by externalnoises, a product of the third output voltage variation ΔVout3 of theoutput voltage Vo and the internal capacitor Cf will be equal to aproduct of the capacitor Cext and a voltage variation Δ Vnoise caused bynoise interference, namely the third output voltage variation ΔVout3 ofthe output voltage Vo will be equal to [(ΔVnoise)×(Cext)]/(Cf).

Then, please refer to FIG. 4A˜FIG. 4C. FIG. 4A˜FIG. 4C illustratedifferent embodiments of the self-capacitive touch sensing circuitrespectively.

In the embodiment shown in FIG. 4A, the self-capacitive touch sensingcircuit not only includes the circuit shown in FIG. 3, but also includesan analog-to-digital converter ADC, a digital signal processor DSP, afirst polarity unit X(1) and a second polarity unit X(−1). The firstpolarity unit X(1) and the second polarity unit X(−1) are coupled inparallel between the output terminal of the operational amplifier OP andthe input terminal of the analog-to-digital converter ADC. The outputterminal of the analog-to-digital converter ADC is coupled to the inputterminal of the digital signal processor DSP.

The first polarity unit X(1) is used for receiving the output voltage Vohaving analog form from the output terminal of the operational amplifierOP and then outputting the output voltage Vo to the input terminal ofthe analog-to-digital converter ADC by maintaining the polarity of theoutput voltage Vo. The second polarity unit X(−1) is used for receivingthe output voltage Vo having analog form from the output terminal of theoperational amplifier OP and then outputting the output voltage to theinput terminal of the analog-to-digital converter ADC by reversing thepolarity of the output voltage Vo. When the analog-to-digital converterADC receives the output voltage Vo having analog form and maintainedpolarity outputted by the first polarity unit X(1) and the outputvoltage Vo having analog form and reversed polarity outputted by thesecond polarity unit X(−1) respectively, the analog-to-digital converterADC will convert them into the output voltages Vo having digital formand then output the output voltages Vo having digital form to thedigital signal processor DSP to perform digital signal processing onthem.

In the embodiment shown in FIG. 4B, the input terminal of theanalog-to-digital converter ADC is coupled to the output terminal of theoperational amplifier OP. The first polarity unit X(1) and the secondpolarity unit X(−1) are coupled in parallel between the output terminalof the analog-to-digital converter ADC and the input terminal of thedigital signal processor DSP.

The analog-to-digital converter ADC is used to receive the outputvoltage Vo having analog form from the output terminal of theoperational amplifier OP and then convert the output voltage Vo havinganalog form into the output voltage Vo having digital form. The firstpolarity unit X(1) is used for receiving the output voltage Vo havingdigital form which is converted from analog form by theanalog-to-digital converter ADC and then outputting the output voltageVo having digital form to the input terminal of the digital signalprocessor DSP by maintaining the polarity of the output voltage Vo. Thesecond polarity unit X(−1) is used for receiving the output voltage Vohaving digital form which is converted from analog form by theanalog-to-digital converter ADC and then outputting the output voltageVo having digital form to the digital signal processor DSP by reversingthe polarity of the output voltage vo. When the digital signal processorDSP receives the output voltage Vo having digital form and maintainedpolarity outputted by the first polarity unit X(1) and the outputvoltage Vo having digital form and reversed polarity outputted by thesecond polarity unit X(−1) respectively, the digital signal processorDSP will perform digital signal processing on them respectively.

In the embodiment shown in FIG. 4C, the input terminal of theanalog-to-digital converter ADC is coupled to the output terminal of theoperational amplifier OP. The input terminal of the digital signalprocessor DSP is coupled to the output terminal of the analog-to-digitalconverter ADC. The first polarity unit X(1) and the second polarity unitX(−1) are coupled in parallel between the output terminal of the digitalsignal processor DSP and the input terminal of another digital signalprocessor DSP.

The analog-to-digital converter ADC is used to receive the outputvoltage Vo having analog form from the output terminal of theoperational amplifier OP and convert the output voltage Vo having analogform into the output voltage Vo having digital form and then output theoutput voltage Vo having digital form to the digital signal processorDSP to perform digital signal processing on it.

The first polarity unit X(1) is used for receiving the output voltagehaving digital form which is converted from analog form by theanalog-to-digital converter ADC and then processed by the digital signalprocessor DSP in order from the output terminal of the digital signalprocessor DSP and then maintaining the polarity of the output voltage Voand outputting the output voltage Vo to the input terminal of anotherdigital signal processor DSP. The second polarity unit X(−1) is used forreceiving the output voltage Vo having digital form which is convertedfrom analog form by the analog-to-digital converter ADC and thenprocessed by the digital signal processor DSP in order from the outputterminal of the digital signal processor DSP and then reversing thepolarity of the output voltage Vo and outputting the output voltage Voto the input terminal of another digital signal processor DSP.

When the another digital signal processor DSP receives the outputvoltage Vo having digital form and maintained polarity from the firstpolarity unit X(1) and receives the output voltage Vo having digitalform and reversed polarity from the second polarity unit X(−1)respectively, the another digital signal processor DSP will performdigital signal processing on them respectively.

It should be noticed that, in the above-mentioned embodiments, everytime when the polarity of the external charging voltage changes, such asswitching from the first external charging voltage (V1) to the secondexternal charging voltage (−V1), the polarity of the output voltage Vowill be also changed.

Under the odd-numbered phase (e.g., the first phase, the third phase, .. . ), when the first external charging voltage (V1) having positivepolarity charges the capacitor Cext, the output voltage variation causedby touch and noise interference will be equal to[(ΔCfinger)×(V1)/(Cf)]+[(ΔVnoise)×(Cext)/(Cf)]; when the second externalcharging voltage (−V1) having negative polarity charges the capacitorCext, the output voltage variation caused by touch and noiseinterference will be equal to[(ΔCfinger)×(−V1)/(Cf)]+[(ΔVnoise)×(Cext)/(Cf)]. If the scan frequencyis much larger than the noise frequency during the scanning process, thetimes (e.g., N/2) that the first external charging voltage (V1) havingpositive polarity charges the capacitor Cext is equal to the times(e.g., N/2) that the second external charging voltage (−V1) havingnegative polarity charges the capacitor Cext, then the total outputvoltage variation caused by touch and noise interference will be equalto(N/2)×{[(ΔCfinger)×(V1)/(Cf)]+[(ΔVnoise)×(Cext)/(Cf)]}−(N/2)×{[(ΔCfinger)×(−V1)/(Cf)]+[(ΔVnoise)×(Cext)/(Cf)]}=N×V1×(ΔCfinger)/(Cf).

It should be noticed that, in the embodiment shown in FIG. 4A, thesecond polarity unit X(−1) will reverse the polarity of the analogoutput voltage Vo; in the embodiments shown in FIG. 4B and FIG. 4C, thesecond polarity unit X(−1) will reverse the polarity of the digitaloutput voltage Vo. That is to say, the output voltage polarity reversingmechanism in the invention can be performed on the analog signal or thedigital signal.

In addition, since the above-mentioned output voltage polarity reversingmechanism has limited noise suppressing effect on the noises at the samefrequency or similar frequencies, the above-mentioned output voltagepolarity reversing mechanism should further dynamically ornon-dynamically adjust the phase switching frequency by cooperating withenvironment detection technology. For example, under the odd-numberedphase (e.g., the first phase, the third phase, . . . ), there should beno limitation to the times, orders or correlations for the firstexternal charging voltage (V1) having positive polarity and the secondexternal charging voltage (−V1) having negative polarity to charge thecapacitor Cext. Therefore, different coefficient combinations can beused to adjust them based on practical needs to achieve the best noisesuppression effect.

Then, please refer to FIG. 5. As shown in FIG. 5, the upper curve is afrequency response curve under the condition having external noises at10 KHz frequency and the lower curve is a frequency response curve underthe condition without external noises. Compared with the prior art shownin FIG. 2, it can be found from FIG. 5 that under the condition havingexternal noises at 10 KHz frequency, the self-capacitive touch sensingsignal obtained by the self-capacitive touch sensing circuit of theinvention not only includes the signal part at the DC level, but alsoextends its low-frequency part at 10 KHz frequency to the high-frequencyregion at 240 KHz; therefore, the frequency range of the noise part ofthe self-capacitive touch sensing signal and the frequency range of thesignal part of the self-capacitive touch sensing signal can be easilyseparated, and the noise part of the self-capacitive touch sensingsignal can be filtered out through the simple low-pass filter.

Then, please also refer to FIG. 6 and FIG. 7. FIG. 6 illustratesschematic diagrams of comparing the self-capacitive touch sensingsignals filtered by the simple one-stage low-pass filter in theinvention and the prior art respectively. FIG. 7 illustrates schematicdiagrams of comparing the performance difference between theself-capacitive touch sensing signals with time in the invention and theprior art respectively.

Obviously, the noise part of the self-capacitive touch sensing signal atlow-frequency region cannot be filtered in the prior art; on thecontrary, the noise part of the self-capacitive touch sensing signal inthe invention is moved to high-frequency region and can be effectivelyfiltered to achieve the noise suppressing effect. It should be noticedthat the band of the noise moved to high-frequency region is (signalreversing frequency+noise frequency/signal reversing frequency−noisefrequency). The invention moves the noise part of the self-capacitivetouch sensing signal from the low-frequency region to the high-frequencyregion and then uses the low-pass filter to filter out the noise part atthe high-frequency region; therefore, the noise removing performance ofthe invention will be much better than that of the prior art.

In addition, although only two-stage voltage levels (e.g., the firstexternal charging voltage (V1) and the second external charging voltage(−V1)) are switched in the above-mentioned embodiments, in fact,multiple-stage voltage levels (e.g., three-stage voltage levels or more)can be also used based on practical needs without any limitations.

Another embodiment of the invention is a noise suppressing methodapplied to a self-capacitive touch panel. In this embodiment, the noisesuppressing method is used to suppress an external environmental noisewhen the self-capacitive touch panel senses a capacitance changingsignal generated by touch.

Please refer to FIG. 8. FIG. 8 illustrates a flowchart of the noisesuppressing method in this embodiment. As shown in FIG. 8, the noisesuppressing method includes the following steps of:

Step S10: instantly sensing a raw band of the external environmentalnoise, wherein the raw band is near a direct current (DC) band of thecapacitance changing signal;

Step S12: selecting a corresponding capacitance driving modulationcoefficient according to a sensing result of Step S10 to move theexternal environmental noise from the raw band to a high-frequency band,wherein a frequency of the high-frequency band is higher than that ofthe raw band and the DC band;

Step S14: pulling the capacitance changing signal back to the DC bandthrough a demodulation mechanism to make the capacitance changing signalin the DC band separated from the external environmental noise in thehigh-frequency band; and

Step S16: using a low-pass filter to filter out the externalenvironmental noise in the high-frequency band to keep the capacitancechanging signal in the DC band.

In practical applications, the noise suppressing method can also switcha first external charging voltage or a second external charging voltageto charge a capacitor according to a specific order, wherein the firstexternal charging voltage is higher than the second external chargingvoltage, but not limited to this.

When the first external charging voltage charges the capacitor, a firstcharge amount stored in the capacitor is a product of a capacitance ofthe capacitor and the first external charging voltage; when the secondexternal charging voltage charges the capacitor, a second charge amountstored in the capacitor is a product of the capacitance of the capacitorand the second external charging voltage.

Compared to the prior art, the invention provides a self-capacitivetouch sensing circuit and a noise suppressing method applied to aself-capacitive touch panel to move a self-capacitive sensing signal toa band having less environmental noise through a driving way ofinstantly adjusting modulation coefficient and then modulate theself-capacitive sensing signal to the DC band and use a simple one-stagelow-frequency filter to filter the self-capacitive sensing signal, sothat the signal-to-noise ratio of the self-capacitive sensing signal canbe effectively increased. The self-capacitive touch sensing circuit andthe noise suppressing method of the invention have the followingadvantages:

(1) Suitable for high noise capacitance sensing environment;

(2) Effectively reducing capacitance driving time;

(3) Effectively reducing entire power consumption for capacitancedriving;

(4) Achieving better touch sensing effect.

With the example and explanations above, the features and spirits of theinvention will be hopefully well described. Those skilled in the artwill readily observe that numerous modifications and alterations of thedevice may be made while retaining the teaching of the invention.Accordingly, the above disclosure should be construed as limited only bythe metes and bounds of the appended claims.

What is claimed is:
 1. A self-capacitive touch sensing circuit, appliedto a self-capacitive touch panel, for sensing a capacitance variation ofa capacitor when the self-capacitive touch panel is touched andsuppressing an external environmental noise, the self-capacitive touchsensing circuit comprising: an operational amplifier having a firstinput terminal, a second input terminal and an output terminal, whereinthe first input terminal is coupled to the capacitor and the secondinput terminal is coupled to ground, and the output terminal outputs anoutput voltage; an internal capacitor coupled between the outputterminal and the first input terminal of the operational amplifier; afirst switch having a terminal coupled to a first external chargingvoltage and another terminal coupled between the capacitor and the firstinput terminal; a second switch having a terminal coupled to a secondexternal charging voltage and another terminal coupled between thecapacitor and the first input terminal; an analog-to-digital convertercoupled to the output terminal of the operational amplifier; a digitalsignal processor coupled to the analog-to-digital converter; a firstpolarity unit, coupled between the output terminal of the operationalamplifier and the analog-to-digital converter, for receiving the outputvoltage having analog form from the output terminal of the operationalamplifier and then outputting the output voltage to theanalog-to-digital converter by maintaining a polarity of the outputvoltage; and a second polarity unit, coupled between the output terminalof the operational amplifier and the analog-to-digital converter, forreceiving the output voltage having analog form from the output terminalof the operational amplifier and then outputting the output voltage tothe analog-to-digital converter by reversing the polarity of the outputvoltage; wherein the first external charging voltage is higher than thesecond external charging voltage; the first switch and the second switchare switched according to a specific order, so that the first externalcharging voltage or the second external charging voltage charges thecapacitor.
 2. The self-capacitive touch sensing circuit of claim 1,wherein the another terminal of the first switch and the anotherterminal of the second switch are both coupled to a node between thecapacitor and the first input terminal.
 3. The self-capacitive touchsensing circuit of claim 2, further comprising: a third switch coupledbetween the node and the first input terminal.
 4. The self-capacitivetouch sensing circuit of claim 3, wherein under an odd-numbered phase,the first switch is conducted but the second switch and the third switchare not conducted, then the first external charging voltage charges thecapacitor, and a first charge amount stored in the capacitor is aproduct of a capacitance of the capacitor and the first externalcharging voltage.
 5. The self-capacitive touch sensing circuit of claim4, wherein under an even-numbered phase, the third switch is conductedbut the first switch and the second switch are not conducted, then theoutput voltage is a quotient of the first charge amount divided by acapacitance of the internal capacitor.
 6. The self-capacitive touchsensing circuit of claim 5, wherein when the capacitor is touched, aproduct of a first output voltage variation of the output voltage andthe internal capacitor is equal to a product of the first externalcharging voltage and the capacitance variation when the capacitor istouched.
 7. The self-capacitive touch sensing circuit of claim 3,wherein under an odd-numbered phase, the second switch is conducted butthe first switch and the third switch are not conducted, then the secondexternal charging voltage charges the capacitor, and a second chargeamount stored in the capacitor is a product of a capacitance of thecapacitor and the second external charging voltage.
 8. Theself-capacitive touch sensing circuit of claim 7, wherein under aneven-numbered phase, the third switch is conducted but the first switchand the second switch are not conducted, then the output voltage is aquotient of the second charge amount divided by a capacitance of theinternal capacitor.
 9. The self-capacitive touch sensing circuit ofclaim 8, wherein when the capacitor is touched, a product of a secondoutput voltage variation of the output voltage and the internalcapacitor is equal to a product of the second external charging voltageand the capacitance variation when the capacitor is touched.
 10. Aself-capacitive touch sensing circuit, applied to a self-capacitivetouch panel, for sensing a capacitance variation of a capacitor when theself-capacitive touch panel is touched and suppressing an externalenvironmental noise, the self-capacitive touch sensing circuitcomprising: an operational amplifier having a first input terminal, asecond input terminal and an output terminal, wherein the first inputterminal is coupled to the capacitor and the second input terminal iscoupled to ground, and the output terminal outputs an output voltage; aninternal capacitor coupled between the output terminal and the firstinput terminal of the operational amplifier; a first switch having aterminal coupled to a first external charging voltage and anotherterminal coupled between the capacitor and the first input terminal; asecond switch having a terminal coupled to a second external chargingvoltage and another terminal coupled between the capacitor and the firstinput terminal; an analog-to-digital converter coupled to the outputterminal of the operational amplifier; a digital signal processorcoupled to the analog-to-digital converter; a first polarity unit,coupled to the digital signal processor, for receiving the outputvoltage having digital form converted from analog form by theanalog-to-digital converter and processed by the digital signalprocessor and then maintaining a polarity of the output voltage; and asecond polarity unit, coupled between the analog-to-digital converterand the digital signal processor, coupled to the digital signalprocessor, for receiving the output voltage having digital formconverted from analog form by the analog-to-digital converter andprocessed by the digital signal processor and then reversing thepolarity of the output voltage.
 11. The self-capacitive touch sensingcircuit of claim 10, further comprising: another digital signalprocessor, coupled to the first polarity unit and the second polarityunit respectively, for receiving the output voltage maintaining polarityfrom the first polarity unit and receiving the output voltage reversingpolarity from the second polarity unit respectively.
 12. Aself-capacitive touch sensing circuit, applied to a self-capacitivetouch panel, for sensing a capacitance variation of a capacitor when theself-capacitive touch panel is touched and suppressing an externalenvironmental noise, the self-capacitive touch sensing circuitcomprising: an operational amplifier having a first input terminal, asecond input terminal and an output terminal, wherein the first inputterminal is coupled to the capacitor and the second input terminal iscoupled to ground, and the output terminal outputs an output voltage; aninternal capacitor coupled between the output terminal and the firstinput terminal of the operational amplifier; a first switch having aterminal coupled to a first external charging voltage and anotherterminal coupled between the capacitor and the first input terminal; asecond switch having a terminal coupled to a second external chargingvoltage and another terminal coupled between the capacitor and the firstinput terminal; an analog-to-digital converter coupled to the outputterminal of the operational amplifier; a digital signal processorcoupled to the analog-to-digital converter; a first polarity unit,coupled between the analog-to-digital converter and the digital signalprocessor, for receiving the output voltage having digital formconverted from analog form by the analog-to-digital converter and thenoutputting the output voltage to the digital signal processor bymaintaining a polarity of the output voltage; and a second polarityunit, coupled between the analog-to-digital converter and the digitalsignal processor, for receiving the output voltage having digital formconverted from analog form by the analog-to-digital converter and thenoutputting the output voltage to the digital signal processor byreversing the polarity of the output voltage.